The present invention relates to alkoxysilane-functionalized allophanates, to a method for production thereof, and to the use thereof.
Polyurethanes have been established for many decades as high-value components for paint, adhesive, sealant and plastics systems. It is possible here for additional alkoxysilane groups to play an important role, for example with regard to network density, chemical resistance and scratch resistance, primarily through the formation of siloxane and polysiloxane structures.
Molecules both possessing alkoxysilane groups and having isocyanate groups offer the option of introducing the functionalities that are the resulting reaction products, siloxanes and polyurethane groups, by means of one component. Such substances have long been in use, for example in the form of isocyanatoalkyltrialkoxysilanes.
Alkoxysilane-terminated polyurethanes prepared from isocyanatoalkyltrialkoxysilanes and alcohols are also known and are used, for example, for producing highly crosslinked, rigid coating compositions (e.g. WO 2013/189882 A2). However, if these alkoxysilane-terminated polyurethanes are used as sole binder in systems that cure at room temperature, coatings are obtained with only moderate hardness.
There is therefore a need for novel binders that overcome the disadvantage of the prior art.
Allophanate-containing binders have long been known. Alkoxysilane-functionalized allophanates are also known. There are several types that can be distinguished here, which are shown below, but correspond neither in terms of structure nor the application to the alkoxysilane-functionalized allophanates according to the invention.
For instance, the allophanates III (1) described in WO 2008/043722 A1 are obtained by reacting NCO-terminated allophanate-containing polyurethanes I (1) with alkoxysilanes II (1) reactive to isocyanate (e.g. aminoalkyltrialkoxysilane). The allophanate groups here are therefore in the centre of the polyurethane chain and the alkoxysilane function is attached via the terminal isocyanate group in the context of a urea function (structure III (1), equation 1).

DE 102005041953 A1 describes the reaction of a polyol I (2) having an average molecular weight of 3000-20000 g/mol with an excess of isocyanatopropyltrimethoxysilane II (2), so as to result in formation of an allophanate IV (2) having two alkoxysilane functions per allophanate unit after the polyurethane formation III (2).

In DE 102005041954 A1, a polyurethane I (3) is treated with isocyanatopropyltrimethoxysilane II (3) and heated, until allophanate structures are formed. In this case, the alkoxysilane group is attached to the terminal nitrogen of the allophanate group (III) (3) (equation 3).

J. Kozakiewicz et al. in Progress in Organic Coatings 72 (2011) 120-130 published the reaction of isocyanatopropyltrimethoxysilane I (4) with methanol, to give the corresponding urethane II (4), and subsequently with hexamethylene diisocyanate trimer III (4). In the highly viscous allophanate IV (4) resulting therefrom, the alkoxysilane function is appended on the tertiary central amine of the allophanate group (equation 4).

In the use described, the allophanate function serves as blocking agent for the hexamethylene diisocyanate trimer which was used as crosslinker for hydroxy-functionalized polyester polyols.
Even now, there exists a need for novel, silane-containing binders which have specific properties.